On Wed, 30 Sep 2020 19:21:59 -0400 "Mark Filipak (ffmpeg)" <markfili...@bog.us> wrote:
> Nyquist [adjective]: 1, Reference to the Nyquist-Shannon sampling > theorem. 2, The principle [1] that, to most faithfully reproduce an > image at a given digital display's resolution, the samples must be > made at or above twice the display's resolution, both horizontally > & vertically [2]. Sorry, but this is wrong. from https://en.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_sampling_theorem "If a function x(t) contains no frequencies higher than B hertz, it is completely determined by giving its ordinates at a series of points spaced 1/(2B) seconds apart. A sufficient sample-rate is therefore anything larger 2B samples per second." Let's say we have 640 horisontal dots (pixels) per line in NTSC system. So imagine NTSC analogue (CRT) camera that shots image of thin alternating black and white vertical stripes. Let say there is 320 white and 320 black stripes, 640 stripes total. This picture produces maximum possible frequency in analogue video signal. One TV line is 64uS. Blanking interval is 12uS. Visible line is 64-12=52uS. This interval in our case is filled with square (ideally) wave. So what is period of this wave? It is of two of those vertical stripes. Onle line is black (negative half-wave) and one white (positive half-wave). So what is frequency of that signal? 1/(52*10^-6)*640/2=6153846Hz or 6.15Mhz and this is our B (Real analogue B/G TV system has slightly lower bandwidth) According to Nyquist-Shannon we must sample at 2B or 12.3Mhz. But what is 2B? It is one sample per one vertical stripe (black or white), or one image dot (pixel). 640 dots per line we want to reproduce = 640 samples per line we must take So it is one sample per pixel. _______________________________________________ ffmpeg-user mailing list ffmpeg-user@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-user To unsubscribe, visit link above, or email ffmpeg-user-requ...@ffmpeg.org with subject "unsubscribe".
